If a car gets 15 miles per gallon of gasoline and can travel 250 miles on a full tank, then the capacity of the fuel tank can be calculated as:

Fuel tank capacity = Miles traveled / Miles per gallon = 250 miles / 15 miles per gallon = 16.67 gallons

Now, if the same car got 20 miles per gallon, then the distance it could travel on a full tank can be calculated as:

Miles traveled = Fuel tank capacity x Miles per gallon = 16.67 gallons x 20 miles per gallon = 333.33 miles

Therefore, if the same car got 20 miles per gallon, it could travel 333.33 miles on a full tank.